A need exists for aircraft engine propulsion nozzles with reduced acoustic noise signatures for low speed operations around airports and the surrounding community. Recently more stringent FAR36 Stage 4 regulations for community noise standard were enabled which further restrict allowable noise emissions from aircraft. The major source of noise from aircraft is the high speed jet emanating from the exhaust nozzles of turbofan engines. Past experimental work established the potential for high radius plug nozzles to reduce jet noise with relatively modest reductions in thrust efficiency. See “Initial Results of a Porous Plug Nozzle for Supersonic Jet Noise Suppression”, by Maestrello, NASA TM-78802, 1978, and “Jet Noise Suppression by Porous Plug Nozzles” by Bauer, Kibens, and Wlezian, NASA Contractor report 3613, 1982.
A high radius plug nozzle consists typically of a circular annulus outer ring 10 on the nacelle exit surrounding a concentric circular inner “plug” shape 11 of smaller diameter, as illustrated in FIG. 1. The exhaust flows out the gap 12 between inner and outer surfaces, with the inner plug lengthwise extending surface generally being several plug diameters in length.
“High radius” refers to a relatively high ratio of the radius of the inner plug relative to the outer cowl radius at the exit plane 13. A high radius plug features a relatively large plug and thin exit annular opening. One of the phenomena leading to reduced noise relative to a simple circular nozzle is the greater shear area relative to the thickness of the annular jet, leading to shorter mixing length between the high speed jet and the relatively lower speed external flow (due aircraft flight speed) passing around the outside of the nozzle and joining at the exit plane. Much of the acoustic work on plug nozzles has focused on “porous” plugs which feature a hollow plug drilled with arrays of small holes, this to suppress “screech” tones associated with shock cells of high pressure ratio supersonic aircraft nozzles. Test results, have also showed significant noise reductions with solid plugs at the lower pressure ratios associated with subsonic aircraft.
Some recent work on jet acoustics has focused on inducing non-circular shaping of the exhaust plumes of simple (non-plug) nozzles, such as by two-dimensional (2-D) rectangular or elliptical cross-sections, or by cutting the nozzle along an angled plane not normal to the direction of flow—a “bevel nozzle”. It has been shown that under favorable circumstances elongating the cross-section of the plume behind these designs can produce a directivity pattern to the noise emissions, typically being reduced in the direction of the elongation of the plume.